Communication devices such as wireless devices may be also known as e.g. user equipments (UEs), mobile terminals, wireless terminals and/or mobile stations. A wireless device is enabled to communicate wirelessly in a cellular communications network, wireless communications system, or radio communications system, sometimes also referred to as a cellular radio system or cellular network. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network. The wireless device may further be referred to as a mobile telephone, cellular telephone, laptop, Personal Digital Assistant (PDA), tablet computer, just to mention some further examples. The wireless device may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile device, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless device or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area is served by at least one base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. Cells may overlap so that several cells cover the same geographical area. By the base station serving a cell is meant that the radio coverage is provided such that one or more wireless devices located in the geographical area where the radio coverage is provided may be served by the base station. One base station may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the wireless device within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile). In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or eNBs, may be directly connected to other base stations and may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for wireless devices. High Speed Packet Access (HSPA) is an amalgamation of two mobile telephony protocols, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), defined by 3GPP, that extends and improves the performance of existing 3rd generation mobile telecommunication networks utilizing the WCDMA. Moreover, the 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies, for example into evolved UTRAN (E-UTRAN) used in LTE.
The expression downlink (DL) is used for the transmission path from the base station to the wireless device. The expression uplink (UL) is used for the transmission path in the opposite direction i.e. from the wireless device to the base station.
The so called Peak-to-Average Ratio (PAR) may be measured by, and also referred to, by the so called Peak-to-Average Power Ratio (PAPR) or Crest Factor (CF). PAPR and CF are calculated from the peak amplitude of a waveform divided by the Root Mean Square (RMS) value of the waveform. A too high PAR when transmitting a radio signal is undesirable e.g. since it sets strict and tough requirements on the power amplifier used when forming the radio signal and results in high power consumption. High PAR is considered a major drawback of multicarrier transmissions such as in the case of Radio Access Technologies (RATs) based on Orthogonal Frequency Division Multiplexing (OFDM) or Discrete Multitone Modulation (DMT) as e.g. is the case for LTE. However, technologies like UTRA and E-UTRA, as used in LTE, were specified with sufficient Error Vector Magnitude (EVM) margins to allow for PAR reduction since it was realized that it would be inefficient to dimension the radio hardware for the peak distributions with high PAR. In this regard UTRA and E-UTRA is quite similar, but GSM was not specified like this, so the same approach may not be taken.
The prior art regarding PAR reduction may be divided into two categories.
1) Methods that influence the PAR of the signal by adjusting various baseband properties. Known methods include e.g. affect coding, the constellation map, or apply reserved frequencies that is used to counter peaks.
2) Methods that know and care very little about the origin of the signal, which are methods such as clipping and filtering, peak windowing or FFT-based schemes. These methods are typically of greatest interest to use in practice since multiple digital signals, from e.g. different units, then may be fed into a single radio unit that performs the PAR reduction for all of the signals in combination. However, the PAR reduction distorts the signal and PAR reduction may therefore have the negative side effect of making it more difficult to fulfill signal quality and emission requirements at least in parts of the spectrum. Hence, most of the methods are combined with distortion shaping, that also may be named noise shaping, typically by filtering and/or windowing designed so that the resulting signal meet requirements such as regarding spectrum unwanted emissions. Distortion shaping alters the spectral shape of the distortion that is introduced by the PAR reduction. The distortion shaping may e.g. decrease distortion at frequencies where this is desirable, e.g. needed to meet requirements, to the expense of increased distortion at other frequencies but where this may be less of a problem. Methods according to category 2 are for example disclosed in:
US20040203430, which relates to peak power reduction using windowing and filtering. A circuit is disclosed that combined a scaling window peak reducing unit and a filter, which may effectively reduce the signal peaks without significantly increasing spurious emissions.
U.S. Pat. No. 7,889,798B2, which relates to a method of reducing the peak-to-mean ratio of a multi-carrier. A residual signal is generated from the multicarrier signal, the residual signal representing the difference between the multicarrier signal and a hard-clipped multicarrier signal. The method also includes the steps of applying a least squares function to the residual signal for each carrier of the multi-carrier signal, thereby generating a minimized residual signal for each carrier and combining the minimized residual signals and the multicarrier signal.
WO2008069488A1, which relates to reduction of PAPR in an OFDM system. A time-domain clipping reduces PAPR. A Fast Fourier Transform (FFT) is performed on the result and then a frequency-domain clipping reduces distortions generated by the time-domain clipping.
Vaananen, O.; Vankka, J.; Halonen, K., “Reducing the Peak to Average Ratio of Multicarrier GSM and Edge Signals”, Electronic Circuit Design Laboratory, Helsinki Univ. of Technology, Espoo, Finland, Personal, Indoor and Mobile Radio Communications, 2002, The 13th IEEE International Symposium, Publication Date Sep. 15-18, 2002, vol. 1, pp. 115-119. In this paper it is disclosed an investigation regarding PAR reduction by clipping for two cases, GSM and EDGE.